lit.cfg: enforce that swift compiler sources are enabled on Darwin and Linux platforms

This is the ensure that we don't accidentally turn of swift compiler sources on those platforms

Author: eeckstein

test/lit.cfg

@@ -276,6 +276,9 @@ if bootstrapping_mode is not None:
     config.available_features.add('bootstrapping_mode')
   elif bootstrapping_mode == 'BOOTSTRAPPING-WITH-HOSTLIBS':
     config.available_features.add('bootstrapping_with_hostlibs_mode')
+else:
+  assert platform.system() != 'Darwin' and platform.system() != 'Linux', \
+    "swift_in_compiler is required for Darwin and Linux"
 
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